Massive resources have been expended on the development of potential therapies aimed at reversing the hypovolemic that is common to different manifestations of systemic inflammatory response syndrome (SIRS). Sepsis alone accounts for 750,000 cases per year in the United States, resulting in 200,000 deaths (1). This high mortality results from multi organ dysfunction (MODS), which is associated with organ edema secondary to capillary leak (CL). Patients with significant CL are typically managed by administering resuscitation fluids containing osmolytes (e.g., albumin, starches, or dextrans) in addition to vasopressors and other supportive measures.
Capillary leak, which is present in different conditions such as multiorgan dysfunction (MODS), sepsis, trauma, burn, hemorrhagic shock, post-cardiopulmonary bypass, pancreatitis and systemic capillary syndrome, causes morbidity and mortality among a large number of hospital patients. Capillary leak (CL) is a central component of MODS, secondary to severe sepsis and systemic inflammatory response syndrome (SIRS). It is characterized by increased capillary permeability resulting in interstitial edema and decreased tissue perfusion leading ultimately to organ failure and death. The leak aspect of capillary leak syndrome (CLS) is reflected in both the release of water into the interstitial space and high molecular weight components of serum which ordinarily would be retained within the capillaries.
Hypovolemic states often lead to hypoperfusion of vital organs, causing organ dysfunction and ultimately resulting in morbidity and death (2). Hypovolemia can occur either rapidly, as with hemorrhagic shock, or progressively due to an underlying disease, with both types involving a systemic inflammatory process. In hemorrhagic shock, hypovolemia occurs due to a rapid and sudden loss of intravascular volume. Upon resuscitation, an inflammatory process may be triggered in reperfused tissues (ischemic-reperfusion injury) causing endothelial cell (EC) injury and capillary leak (CL) leading to a secondary hypovolemic state. In sepsis and other diseases, systemic inflammation is triggered by the disease and in a similar sequence leads to EC injury, CL, and ultimately hypovolemic shock.
Resuscitation with plasma volume expanders remains a mainstay in treating hypovolemia, but with mixed results. The efficacy and safety of volume expanders, including both colloids (e.g., albumin and starches) and crystalloids, continue to be topics of intense research and controversy (3,4). The unpredictable effectiveness of albumin as a plasma expander may be linked to the severity of the underlying EC injury (5). Specifically, if the endothelial integrity is compromised such that albumin can readily extravasate, the leaking albumin may exacerbate the oncotic gradient favoring CL, as opposed to reversing it.
Though the biological mechanisms that induce CL syndrome are poorly understood, some evidence indicates the involvement of inflammatory cytokines. Fluid replacement with solutions of human albumin is only marginally effective since it does not stop the loss of albumin into the extravascular space. Albumin is important because it is responsible for plasma oncotic pressure as well as for retaining sodium ions in the blood.
Under normal conditions, albumin contributes to about 80% of the total blood colloid osmotic pressure (6) and is ideally sized such that it extravasates at a low physiologic rate (7). In CL patients, 5% to 20% albumin solutions are often administered to increase circulating blood volume and to augment intravascular osmotic properties. This method of retarding CL makes the tenuous assumption that albumin can maintain its normally low extravasation rate during shock. Clinical data, however, show that the efficacy of albumin is inconsistent at best (8,9). Some have even suggested that resuscitation with albumin may increase mortality in critically ill patients (10).
PEGylation has been used extensively (11,12). Modification of interferon beta-1a with polyethylene glycol prolongs its half-life, resulting in higher antiviral activity (13). There have been studies on the use of PEGylated hemoglobin (PEG-Hb) as a substitute for blood (14,15,16). Large amounts of PEG-Hb, constituting up to 80% vascular volume showed that PEG-Hb is effective in maintaining the hemodynamics and oxygen delivery in the rat (17). These studies suggest that PEG-Hb is safe even at very high doses.
Other colloids have been used to treat capillary leak conditions with varying degrees of efficacy. A variety of heterogeneous (Mr weighted average: 125,000-450,000 Da) starch colloids have been proposed or are in use as substitute for albumin (18). While these compounds are less expensive and more readily available than pooled human albumin, use of starch colloids has been restricted to low doses due to safety issues that severely limit their use. In addition, the high Mr (>1,000,000 Da) moieties within the heterogeneous starch colloids can alter blood rheological properties and cause coagulopathy (19). The relatively homogeneous Pentastarch (Mr=110,000) has been shown to attenuate lung injury in an aortic occlusion reperfusion injury model (20).
In a recent study, MAP and heart rate (HR) did not change favorably when hetastarch (HES) was given in a septic pre-treatment rat model (21). In contrast, favorable changes in MAP (increased) and HR (decreased) were observed in rats pre-treated with polymerized hemoglobin. This occurred despite the fact that, at the same molar concentrations, the colloid osmotic pressure of HES (27 mm/Hg) was higher than the polymerized hemoglobin (21 mm/Hg). Use of the latter as a routine plasma expander is however controversial and is complicated by potential side effects particularly in relation to the kidneys.
Finally, several studies have suggested that albumin has an endothelial anti-apoptotic effect by mediating regulation of cellular glutathione and nuclear Factor Kappa B activation (22,23,24). This may play a significant role in sepsis induced CL particularly in light of a recent report that linked CL in different systemic inflammatory response manifestations to endothelial cell apoptosis (25).
The available albumin today has a molecular weight of 69,000 with a very short half-life (4-6 hours) which can easily leak to the extravascular space in capillary leak conditions such as severe sepsis, pancreatitis, burn and trauma. This leaking can cause worsening edema and/or compartment syndrome. The use of pentastarch and hexastarch are of limited value since they are not for use in pediatric patients and can cause bleeding. Additionally, only 15 cc/kg can be used in patients. Further, the pentastarch and hexastarch have been shown to cause intractable pruritus (itching) after use and the effect lasted for years. In fact, some studies state that the use of albumin as a replacement or as a volume expander is counterproductive since it increases edema by drawing fluid out of the capillaries.
Therefore, there is a great need for a composition and a method to effectively prevent and/or treat hypovolemic conditions which does not have the above-described disadvantages.